Most of the conventional high-frequency heating appliances use an electron tube oscillator such as a magnetron in their heat source, which electron tube oscillator requires a high voltage, usually thousands of volts, for its drive power source, imperatively needing massive step-up transformers and high voltage capacitors.
With advances in semiconductor technology, solid-state elements capable of high-output function have made their advent in the microwave region, accelerating infusion of solid-state techniques into television relay stations.
In such a background has come the realization of a solid-state version of the high-frequency heating source which is the nucleus of high-frequency heating appliances, providing a high-frequency heating appliance equipped with a low voltage drive solid-state high-frequency generating section in contrast to the conventional electron tube oscillator, dispensing with the massive step-up transformer and high-voltage capacitor, thereby making it possible to design the appliance so that it is lightweight and compact.
Solid-state elements, as is known in the art, have upper limits in their operating frequencies and also in the power they can handle. These two types of limits may be said to be of an opposing nature in terms of element structure; that is, the output power of a solid-state element is inversely proportional to its operating frequency or the square of its operating frequency. As a result, when it is desired to obtain output power equivalent to that of the conventional high-frequency heating appliance, a plurality of solid-state elements must be used to constitute the solid-state high-frequency generating section.
Further, a high-frequency heating appliance is provided with a door for access to the heating chamber to insert and remove an object of heating. The door, which is a component of the heating chamber, in civil or private high-frequency heating appliances, has applied thereto approaches to convenient usage and aesthetic improvement; thus, generally, the door or heating chamber flange is provided with a microwave energy sealing apparatus using a choke seal mechanism or microwave energy absorber. Besides this, the door is provided with a viewing window for looking into the heating chamber, which viewing window is also provided with a means for preventing microwave energy leakage.
Although the door is considerably heavy, there has been no case of the body of a conventional high frequency heating appliance being tilted upon opening and closing of the door, because of the presence of the heavy step-up transformer and the like in the body. However, when the body is made lighter and lighter in weight by the solid-state version of the high-frequency heating source, the balance of the weight between the door and the body is upset, so that there is a danger of the body being tilted upon opening and closing the door.
The width, depth and height of the heating chamber in the conventional high-frequency heating appliance are considerably large, by reason of practical use, for the predetermined wavelength of high-frequency waves from the high-frequency heating source.
As a result, there is produced in the heating chamber a variety of electromagnetic field modes. Some of these electromagnetic field modes are suitable for uniform heating of an object but the rest are not. In selecting a particular electromagnetic field mode suitable for uniformly heating a particular object, there may be contemplated a method of controlling the oscillation frequency of the high-frequency heating source, but the mechanism to implement such a method is expensive.
Therefore, the conventional appliance has resorted to deliberately disturbing the electric field distribution in the heating chamber as by a stirrer, or to rotating an object of heating, but the mechanism therefor is complicated and, moreover, does not necessarily ensure uniform heating.
In connection with a high-frequency heating appliance comprising a high-frequency heating source capable of variable control of oscillation frequency in a predetermined frequency band, and a heating chamber having such a size as to produce particular electromagnetic field modes, we have previously provided a high-performance appliance wherein the oscillation frequency of the high-frequency heating source is variably controlled to cause the heating chamber to resonate with said particular electromagnetic field modes; but, the mechanism for variably controlling oscillation frequency is expensive.